The technique of the fluidized bed is currently employed for the processing of solid granular materials and particularly for the combustion or gasification of solid combustible materials such as coal, lignite, wood and biomass waste.
A fluidized bed processing installation comprises a fluidization reactor formed by an elongated chamber having a vertical axis and provided at its base with means for injecting a fluidization gas with a variable rate of flow. The material is introduced, in the form of particles, in the fluidization chamber above a grating for a homogeneous distribution of the gas. Different fluidization means may be employed.
There is produced in this way inside the fluidization chamber a rising current of gas discharged in the upper part of the chamber and consisting of the fluidization gas and gaseous products issuing from the processing, for example from the combustion of the material. It is known that, in regulating the speed of circulation of the gases, it is possible to operate in accordance with different fluidization rates. The particles start to be put into suspension indeed beyond a minimum fluidization speed. However, if the fluidization speed exceeds a value which may reach, for example, 2 to 6 m/sec and which depends on the particle size and the density of the particles, the major part of the latter is liable to be entrained upwardly with the gases. There is then distinguished inside the fluidization chamber a dense lower zone and a more diluted upper zone which extends up to the upper part of the fluidization chamber, the limit between the two zones being, moreover, indistinct. In its upper part, the fluidization chamber communicates with a circuit for discharging the gases which passes through a separating device of the cyclone type in which the solid particles entrained with the gases are recovered, the latter being discharged, after purification, through an upper outlet of the separator, while the recovered particles accumulate at the base and are discharged through a lower outlet orifice connected to the reactor through a circuit for recycling the recovered particles in the fluidized bed. This recycling circuit is often formed by a conduit in the shape of a syphon or trap in which the particles circulate in a dense form and provide a seal for balancing the pressures between the fluidized bed reactor and the separator.
In the course of their return to the fluidized bed reactor, the recovered particles may be subjected to a processing in an outside device placed either directly in the recycling conduit or branch-connected to the latter. For example, the particles may be passed through a heat exchanger constituted by a vessel in which may be placed a nest of exchange tubes through which a heat-carrying fluid travels. The coefficient of heat transfer is increased if the particles are put into suspension by the circulation of a rising current of fluidization gas introduced in the lower part of the vessel. As the case may be, the particles may be fed at the base of the vessel or in its upper part and may be discharged in the known manner, for example by overflowing toward a recycling conduit which communicates with the reactor.